When extruding aluminium it is important to ensure that the speed of movement of the aluminium through the extrusion die is uniform across the die. This has been achieved in the past using a die having a die cavity of finite bearing length, and by varying the bearing length across the die. It has been found, however, that the extruded product sometimes contains surface imperfections resulting from the engagement between the bearing surface and the aluminium being extruded. Rather than provide the bearing surface downstream of the entrance to the die cavity, it is known to use a die having a so-called zero bearing, and to provide a chamber upstream of the die cavity of varying bearing length to control the extrusion speed over the die. Although the term zero bearing suggests that the die cavity is of zero bearing length, in practise the die cavity is likely to have a finite, but very small bearing length.
Another problem which has been faced when extruding aluminium is that, where the extrusion is, for example, of channel section, the sides of the channel tend to deflect thus, if the die is shaped to include a die cavity in which the parts thereof which form the sides of the channel are parallel to one another, the sides of a member extruded using the die may be splayed, rather than parallel to one another. In order to correct such splaying, it is known to provide a pre-chamber located upstream of the die cavity, the pre-chamber being of greater width than the part of the die cavity immediately adjacent therein the pre-chamber being offset laterally relative to the die cavity. Such a technique results in a side loading being applied to the metal being extruded. Although off-setting the pre-chamber laterally from the die cavity can correct splaying in some circumstances, it is of limited application and may not be able to apply a sufficient force to correct splaying in, for example, extrusions of relatively low wall thickness. Similar problems are experienced when hollow members are extruded using a die comprising a male part and a female part.